1. Field of the Invention
The present invention relates generally to automatic tape laying machines that are used to apply uncured composite material (prepreg) to molds during fabrication of composite parts. More particularly, the present invention is directed to the tape that is used in such machines.
2. Description of Related Art
Composite materials are widely used in applications where high strength and relatively low weight are required. A common procedure for forming composite parts involves applying a prepreg to a mold or other tooling to form an uncured part that is then cured using an autoclave or other suitable curing protocol.
“Prepreg” is a term used in the composite industry to describe a composite precursor wherein one or more layers of fibrous reinforcement have been impregnated with uncured resin. The resulting pre-impregnated structure is typically stored for later use in fabricating the final cured composite structure. The preparation and use of prepregs is particularly desirable in the fabrication of aircraft parts, wind turbine blades and other critical structures because it allows the manufacturer to carefully control the amount of resin that is combined with a given amount of fibrous reinforcement. As a result, the final properties of the cured composite structure can be carefully controlled.
Prepreg may be applied to the mold or tooling in a variety of ways depending upon the size of the part and the surface complexity. Automated prepreg application procedures are particularly desirable for fabricating large composite parts, such as aircraft fuselages, wing skins and wind turbine blades. One such automated process involves the use of automated tape laying machines that are commonly referred to as “automated tape layers”. Automated tape layers tend to be large gantry style machines in which a movable tape delivery head mechanism is mounted between parallel rails. The tape delivery head is generally computer controlled and can be moved about multiple axis to provide delivery of a prepreg tape to a variety of mold shapes. A detailed description of automated tape laying machines is provided in the technical paper by Grimshaw et al. entitled “Advanced Technology Tape Laying For Affordable Manufacturing of Large Composite Structures” (46th international SAMPE Symposium, pp. 2484-2494, May 6-10, 2001).
The prepreg tape that is used in automated tape layers contains a layer of uncured composite material (prepreg) that is supported on a backing that is typically paper. The backing is removed as the tape is placed onto the mold by the delivery head. The tape material is provided as a large roll or spool of tape that is mounted on the machine so that the tape can be fed continually to the delivery head. The tape is typically from 75 mm (3 inches) to 300 mm (12 inches) wide. The tape varies in thickness and weight depending upon the particular prepreg and backing material being used.
The backing is continually wound onto a take up roller after the prepreg has been placed on the mold. As a result, there is continuous tension on the backing between the supply roll, delivery head and the take up roll. The tape is also typically heated at the delivery head and a certain amount of compaction pressure is applied to insure proper adhesion of the prepreg to the mold or to previously applied layers of prepreg. In addition, the machine lays the tape in a computer-controlled path and cuts through the prepreg at precisely controlled locations and angles.
A major goal in the operation of automated tape layers is to provide a backing that does not break during its journey from the supply roll to the take up roll. Stopping and restarting automated tape laying machines due to the breakage of the backing material is a costly and time-consuming operation. Accordingly, it is important to provide a tape backing that has sufficient dimensional stability, tear strength and burst strength to withstand the many forces that are applied to the backing as it travels through the automated tape layer.
The tape backing must not only be strong, but it must also be low weight and resistant to tearing. This is especially important since the cutting blades that are used to cut through the prepreg layer purposely score the backing to ensure the prepreg layer is fully cut through. In addition, the backing must have specific differential release properties. For example, the surface of the backing must not stick to the prepreg as the tape is being unwound from the supply roll. The prepreg must also remain adhered to the backing until it reaches the delivery head where it is differentially released onto the mold or onto previously applied prepreg. The backing must also be resistant to moisture for dimensional stability and in order to prevent possible water-based degradation of the backing when it is in storage, in use and under tension.
Paper-based backing materials, which have been treated with silicone as a release agent, are commonly used with automated tape laying machines. One problem with paper-based backing materials is that they are subject to tearing, especially if they are scored during cutting of the prepeg layer. The paper backing also tends to lose strength if moisture is absorbed when the tape is in storage or if in use and under tension. Attempts to solve these problems have involved increasing the thickness of the paper and reinforcing the paper with an adhesive tape. Neither of these solutions has been entirely satisfactory.
Paper based backing materials are typically slit longitudinally into various widths during the process of making prepreg tape for use in automated tape laying machines. Ideally the paper backing materials will have clean slit edges. A common problem with conventional paper based backing materials is that the resultant slit edge can be rough and contain fibrous debris that is subsequently deposited onto the uncured composite material as a contaminant.
Accordingly, it is desirable to provide new backing materials that are tear resistant, even if they are scored. In addition, backing material should have the other properties mentioned above that are necessary for the backing to be suitable for use in an automated tape layer.